Lubricating compositions in use today are prepared from a wide variety of natural and synthetic base stocks to which have been mixed various additive packages and solvents depending upon the intended field of application. The various additives employed in the additives packages can include one or more additives selected from viscosity index improvers, corrosion inhibitors, oxidation inhibitors, dispersants, lube oil flow improvers, detergents and rust inhibitors, pour point depressants, anti-foaming agents, anti-wear agents, seal swellants, friction modifiers, extreme pressure agents, color stabilizers, demulsifiers, wetting agents, water loss improving agents, bactericides, drill bit lubricants, thickeners or gellants, anti-emulsifying agents, metal deactivators, and additive solubilizers. These additives are added to base stocks such as mineral oils, highly refined mineral oils, poly alpha olefins, polyalkylene glycols, phosphate esters, silicone oils, diesters and polyol esters.
There has been considerable effort expended to develop lubricating oil compositions which will reduce friction and wear in engines, particularly automobile engines, since such reduced friction and wear improves the fuel efficiency of the engine. As a result of these efforts, various friction-modifiers and new reduction additives have been added to lubricating compositions.
A number of oil soluble molybdenum (Mo) compounds have been disclosed as useful in providing desirable lubricating oil properties such as antiwear, antioxidant and friction reduction properties. Among the disclosures of molybdenum compounds for such purposes there may be mentioned U.S. Pat. Nos. 4,164,473; 4,176,073; 4,176,074; 4,192,757; 4,248,720; 4,201,683; 4,289,635; 4,479,883 and Japanese Paten Publication No. 56000896. As an example of such molybdenum compounds, there may be mentioned, molybdenum dithiocarbamate (MoDTC) which has been recognized as providing benefits in all three of the aforesaid areas. Good antiwear and antioxidant benefits as well as some friction modification properties are obtained at molybdenum concentrations of 100 ppm. For good friction modification properties, molybdenum concentration of about 500 ppm are employed.
For improved lubricating oil package stability in engine oils, especially for fully synthetic oils, and also for improved sludge handling, piston cleanliness and antioxidant benefits, small amount of generally from about 5 to about 15 wt % of ester base oils have been employed.
It has previously been disclosed in U.S. Pat. No. 4,479,883 that lubricating oil compositions of somewhat improved friction reducing properties are obtained when a hydroxy substituted soluble ester of a saturated or unsaturated polycarboxylic acid having from 24 to 90 carbon atoms between the carboxylic acid groups and a metal dithiocarbamate such as a molybdenum dithiocarbamate (MoDTC) are both employed in a lubricating oil composition.
However, when an additive of a high hydroxyl ester comprising trimethylolpropane and a C.sub.8 -C.sub.10 acid having about one hydroxyl group per molecule of trimethylolpropane left unconverted is employed as an ester in combination with a MoDTC and added to a lubricating oil composition the end friction coefficient and wear volume properties were not improved, but were found to be generally less favorable than a composition of the lubricating oil and the high hydroxyl ester of trimethylolpropane and a C.sub.8 -C.sub.10 acid.
There continues to be a need for additives that can be added to lubricating base composition to provide significantly enhanced and improved properties in regard to friction coefficients and wear properties.